1. Field of the Invention
This invention relates to a manway lift assist mechanism, a spring actuated lift assist assembly therefor, the lift assist assembly comprising a hinge assembly and flat spiral springs housed within substantially closed cover assemblies for protection from the environment. The number of hinge assemblies required for a manway as well as the number of lift assist assemblies for each hinge assembly is dictated by the weight of the cover and the resultant lifting force required. The lift assist assemblies of this invention produce a torque opposing the torque resulting from the cover weight thus reducing the required lifting force to an ergonomically acceptable value.
2. Prior Art Statement
Traditionally, manways have been used to gain access to the interior of pressurized and non pressurized vessels. The manway consists of a cover and a collar assembly, the collar assembly welded to the top or sidewall of the vessel. To facilitate opening of the manway cover, at least one hinge is typically attached at a point exterior to the manway cover with a lift handle attached to the cover at a point furthest from the hinge. Due to an ever growing concern over industrial ergonomics, better methods of reducing the force required to lift the cover are being sought. Looking at the problem as a function of torque, the weight of the cover in the closed position produces torque based upon the distance from the center of gravity of the cover to the pivot point of the hinge. To open the manway, a lifting force is applied utilizing the handle. This lifting force is applied to produce a torque opposite and greater than that of the torque produced by the weight of the cover, thus opening the manway.
It is known to provide a door closer for mounting on a door member or jamb member comprising a base plate, a plurality of laterally spaced and aligned bearings carried by the base plate, laterally spaced actuator arms interconnected at their outer ends and means carried by the outer arm ends for establishing an operative connection with the other of the door and jamb members, a shaft projected through the plate and the arm bearings and pivotally securing the actuator arms to the base plate in combination with a plurality of its coils springs spaced in the direction of the length of the shaft and surrounding the same. For instance, see the U.S. Pat. No. 2,823,413 issued on Feb. 18, 1958 to Kenneth M. Stewart. The closing force of the door may be changed by lifting an end of one of the springs from the actuator arm, however, external adjustment is otherwise limited without removal of the door closer from the door. Furthermore, the door closer of Stewart is unprotected the from the environment and is subject to damage from environmental factors. Additionally, no latch is incorporated into the door closer in order to retain the door in an open or partially open position. The device of Stewart is inappropriate for a manway cover, therefore, there is a need for a lifting assist mechanism that comprises multiple lifting assist mechanisms, provides for placement of a retaining clamp between the hinges and provides means for locking the manway in the open position.
It is also known to constantly urge a lid into an open position by a pair of helical springs for each hinge pin through which the pin extends. Two separate hinge pins are shown, each hinge pin having a pair of helical springs disposed therearound. In the order to limit the upward movement of the lid to a preselected position, stops are provided. For instance, see the U.S. Pat. No. 3,459,462 issued on Aug. 5, 1969 to Barnard, et al. As with Stewart above, no means is provided externally to adjust the spring force supplied by any of the springs, nor is there a latch to retain the lid in any one of a plurality of positions. Additionally, the device described by Barnard, et al., is also unprotected from environmental factors and therefore cannot be used in rigorous environments such as for a manway cover. A need exists for a lifting assist mechanism that may readily be adjusted for the appropriate lifting force, is protected from the environment and provides means for locking the manway in the open position.
It is further known to provide a balancing aid for pivotally mounting a manhole cover comprising a stack of conical disc spring and means to compress the stack of springs as the cover pivots from an open vertical position to a closed, substantially horizontal position wherein the stack of springs comprises a first subset of springs having a first diameter and a second subset of springs having a different diameter, the first subset of springs having a steeper spring force characteristic curve than the second set whereby the overall spring force characteristic curve of the combined stack of springs as the cover pivots from the open vertical position to the closed position is progressive. For instance, see U.S. Pat. No. 4,137,669 issued on Feb. 6, 1979 to Erwin Nunlist. Though effective in assisting with lifting the manhole cover, failure of any part results in loss of lifting force. Furthermore, though the manhole cover can be balanced in any position, no means of locking the manhole cover in the open position is present. Finally, since the balancing aid is disposed between the hinges, the space between the hinges is completely obscured and placement of a retaining clamp between the hinges is not possible. Therefore, there remains a need for a lifting assist mechanism for a manway cover that comprises multiple lifting assist mechanisms, provides for placement of a retaining clamp between the hinges and provides means for locking the manway in the open position.
Additionally, it is known to provide a retro-fit cover balance assembly comprising a pair of torsion springs having one end passed beneath the cover hinge members and bearing upon the collar hinge member. The through bolt is removed, a spring cup is arranged adjacent each side edge of the cover hinge members, the springs are disposed over the spring cups and the through bolt replaced. Holes for eye bolts are provided in the cover hinge members disposed away from the center of rotation and the free end of the springs are inserted through the eye bolts. Access to retaining clamps would not be possible between the cover hinge members, nor is the balance mechanism protected from the environment, nor is there external adjustment of the spring tension. For instance, see the U.S. Pat. No. 5,394,650 issued on Mar. 7, 1995 to Robert Dean.
Finally, it is known to provide a compression spring which moves a cam member against the underside of the manway cover to help in lifting the manway cover. External adjustment of the spring force is provided by an external bolt head. The spring is housed in an enclosure. Access to a retaining clamp between the cover hinge members would be denied. For instance, see the U.S. Pat. No. 6,446,307 B2 issued on Sep. 10, 2002 to Larry C. Wilkins.